As oil exploration continues in remote locations, the use of offshore drilling techniques and structures will become more commonplace in ice-infested areas. Platforms are continually erected in isolated areas that have extremely severe weather conditions. However, the structures that operate in more temperate climates cannot usually be employed here because they must be able to cope, not only with severe arctic storms and sea ice incursions, but also with large and small icebergs that are driven by wind, current and wave action. Because of these conditions, many different types of platform designs have arisen in an attempt to cope with the harsh weather and other natural elements.
Currently, much exploration is conducted in the arctic and in the ice-infested waters off Alaska, Canada and Greenland. To cope with the iceberg and weather problem, some structures attempt to resist these large ice masses by simply being large enough to withstand the largest conceivable impact forces. Examples of these designs may be seen in dual cone structures, such as U.S. Pat. No. 4,245,929, large reef-like structures, or many other gravity based large concrete-steel configurations, see also U.S. Pat. No. 4,504,172. However, these structures are either very heavy, very expensive, or are permanently affixed to the sea bottom. As such, they do not lend themselves to either reuse or quick site evacuation in the case of an emergency situation. In addition, ultimate removal and abandonment of these structures upon oil field depletion is extremely difficult. Due to the wide variability in iceberg characteristics and lack of data about them, a more problematic issue with these structures concerns the definition of the largest iceberg to design for--the selection of the design iceberg requires a reasonable balance of risks and costs, made difficult by the inherent uncertainties.
Another factor to be considered is cost. Generally, the type of large gravity based structure that may be used for arctic exploration and production is very expensive and time consuming to build. With the unproven nature of some of the oil prospects, the harshness of the environment, the increased costs and delays due to thet weather down time, the probability of failure, and even the political climate, it becomes even more risky for an oil company to invest a large amount of money or time. In the event of an accident or other type of misadventure, losses could be greatly multiplied.
To overcome many of the disadvantages of these previously discussed arctic structures, it would be advantageous to combine some of the principles of the gravity-based structures with those of the floating structures. This is accomplished by constructing a platform that has subsurface hull chambers that may alternatively provide buoyancy or ballast and a subbase upon which the platform may rest. The complete structure may then be towed in a floating mode to an offshore drilling/production site and slowly filled with ballast until both the platform and the subbase rest on the sea floor in a gravity-based mode. When a situation, threatening to the structure, presents itself, the platform may be deballasted back to a floating condition and removed from the site to leave the subbase behind. However, this deballasting procedure is quite slow (on the order of 6 to 7 hours) and since it is probably going to be done in rough seas, there is a large chance that the platform, and/or the subbase on which it rests, may be damaged when it "bounces around" due to wave action as it approaches neutral buoyancy on the subbase and then while it slowly ascends to its final floating draft.
A solution to this problem is to keep the platform on the subbase with a temporary hold-down means while it is being deballasted. Once it has fully deballasted, the hold-down means may then be released to allow the platform to quickly ascend to its floating draft and escape damage.
This hold-down system may be mechanical or hydraulic, however, because a mechanical system: may not assure a simultaneous release of all connection units; is expensive; requires a sophisticated control system; and is difficult to reuse or to replace damaged or used connection units, a hydrostatic sealing system is chosen. This hydrostatic system will hold the platform to the subbase from the beginning of the deballasting procedure to the time when deballasting is complete. After deballasting, the platform may be quickly detached by releasing the hold-down system and then floated away from the impending iceberg danger.
To eliminate most of the problems of these previously mentioned arctic structures for use in iceberg-infested waters, the Removable Bottom Founded Structure (RBFS) concept was developed to provide a platform which may be removably detached on short notice from its subbase and, if necessary, transported to a safer location. Other advantages of this structure include providing: (1) a wellhead protection device (i.e., the subbase) against those icebergs large enough to scour the sea floor, (2) a capacity for a higher deck load than floating structures (as the RBFS rests on the sea bottom in the normal operating mode), (3) the ability to quickly evacuate the platform from its fixed location on the sea floor by deballasting and then releasing the hold-down means, (4) reduced capital costs from the gravity based structures due to a more economical design, (5) greater flexibility in structure siting due to the platform's mobility, (6) direct subsea well access from the fixed deck overhead, (7) protection of the vertical production risers from waves and ice due to their placement within the platform columns, and (8) the ability to relocate most of the structure to a new site if dictated by changing reservoir information (only a new subbase would be required for each relocation).